Fiber lasers, that is lasers made from a fiber optic element doped with a rare earth element such as erbium or neodymium or any other element that will cause optical gain, are appealing because they are single-mode waveguided and have high gain and high efficiency. Since these fiber optic elements are generally silica-based they are inhomogeneous line-broadening gain mediums. Therefore they lase over a broad band of wavelengths: they are not what is referred to as a single-frequency laser.
Thus additional, discrete, external optical components are required to select the desired wavelength or narrow band of wavelengths from all of those generated. Lasers other than fiber lasers do provide narrow bandwidths but they lack the simplicity and advantages of a fiber laser.